The claimed invention relates to novel catalysts for use in processes for the selective oxidation of hydrogen sulfide to form elemental sulfur.
As is known, many gases, both natural and industry generated, contain hydrogen sulfide (H.sub.2 S). For example, the H.sub.2 S content of natural hydrocarbon gases can be up to 25%. Hydrotreater gases, synthesis gases from coal gasification, and the like also contain H.sub.2 S. It is very important to convert the H.sub.2 S into sulfur for many reasons.
One reason is that the presence of hydrogen sulfide in a gas, even in very small quantities, decreases the value of the gas, often making the gas valueless. This is because H.sub.2 S has a noxious smell, is highly corrosive, is an extremely strong poison for most living things, including humans, and is a poison for many catalysts.
Hydrogen sulfide conversion into elemental sulfur (S.sub.x) can be carried out by two different principle methods:
(a) Decomposition, according to reaction: EQU H.sub.2 S.fwdarw.H.sub.2 +1/x S.sub.x ( 1)
(b) Oxidation, according to reactions: EQU H.sub.2 S+ 1/2 O.sub.2 .fwdarw.H.sub.2 O+1/x S.sub.x ( 2) EQU H.sub.2 S+ 1/2 SO.sub.2 .fwdarw.H.sub.2 O+3/2x S.sub.x ( 3)
In the second method, in addition to O.sub.2 and SO.sub.2, other oxidants can be used, such as H.sub.2 O.sub.2, NO.sub.x, and the like.
From a practical point of view, the most attractive way of sulfur production from hydrogen sulfide is selective oxidation by using oxygen from air, according to reaction: EQU H.sub.2 S+ 1/2 O.sub.2 .fwdarw.H.sub.2 O+1/x S.sub.x ( 4)
Reaction (4) is thermodynamically possible over a very wide range of industrially acceptable temperatures. Without a catalyst, however, the rate of reaction is low and the reaction is noticeable only at temperatures higher than 300.degree. C. However, reaction (4) is accompanied by conversion of H.sub.2 S to sulfur dioxide at temperatures greater than about 300.degree. C. by the reactions: EQU H.sub.2 S+ 3/2 O.sub.2 .fwdarw.H.sub.2 O+SO.sub.2 ( 5) EQU 1/x S.sub.x +O.sub.2 .fwdarw.SO.sub.2 ( 6)
Further, sulfur dioxide can form according to the reverse Claus reaction: EQU 3/2x S.sub.x +H.sub.2 O.fwdarw.H.sub.2 S+ 1/2 SO.sub.2 ( 7)
Thus, in order to selectively form elemental sulfur, H.sub.2 S oxidation should be conducted at temperatures less than about 300.degree. C. However, this is only possible by the use of suitable catalyst. A preferred catalyst should not promote the reverse Claus reaction (reaction (7)) to minimize the formation of SO.sub.2. If a solid catalyst is used, the process temperature should be at least 180.degree. C., in order to prevent condensation of formed sulfur on the catalyst. Condensed sulfur blocks the catalyst surface, thereby reducing H.sub.2 S oxidation rate.
In sum, to carry out oxidation of selective H.sub.2 S to S, catalysts showing high activity at temperatures 180.degree.-300.degree. C. are required. In addition to high activity, it is desirable that the catalysts possess high selectivity, because in the reaction medium, which contains H.sub.2 S and O.sub.2, in addition to undesirable side reactions (5) to (7) above, other undesirable side reactions which decrease H.sub.2 S conversion into S are thermodynamically possible. These other undesirable side reactions include: EQU H.sub.2 S+2O.sub.2 .fwdarw.H.sub.2 O+SO.sub.3 ( 8) EQU 1/x S.sub.x + 3/2 O.sub.2 .fwdarw.SO.sub.3 ( 9)
These reactions usually take place only at temperatures higher than about 400.degree. C.
For achievement of highly selective oxidation of H.sub.2 S into S.sub.x by use of a solid catalyst, preferably the catalyst contains as few small pores and as many large pores as possible. This structure allows molecules of formed sulfur to leave catalysts pores rapidly and thereby avoid reactions (6) and (7). Since a catalyst's surface is generally made up of its pores, catalysts with large pores usually do not have a large specific surface since specific surface is inversely proportional to pore diameter.
Different methods for preparing catalysts with large pores and accordingly small specific surface are known in heterogeneous catalysis. For example, USSR Inventors Certificate 871813 (1981) (which is incorporated herein by reference) discloses an iron oxide based catalyst having specific surface 1-2 m.sup.2 /g and average pore diameter 2500-2900 .ANG. for use as a H.sub.2 S oxidation catalyst. USSR Inventors Certificate 967551 (1982) (which is incorporated herein by reference) also discloses a catalyst in which an active compound is applied on an inert carrier having a specific surface of 1.5-2.0 m.sup.2 /g and an average pore diameter of 2500-3000 .ANG.. U.S. Pat. Nos. 4,818,740 and 5,037,629 disclose catalysts prepared by depositing oxides of iron or oxides of iron and chromium on carriers having large pores and small specific surface for the selective oxidation of H.sub.2 S to S.
The pore structure of a catalyst allows the active components of the catalyst to perform effectively. The catalyst pore structure however, by itself, cannot provide high activity and selectivity. These are effected by the chemical and phase composition of the catalyst. Thus, to provide an effective catalyst, chemical and phase composition must be optimized.
However, the level of knowledge of chemistry and catalysis does not allow the prediction of a catalyst composition for a given reaction.
Analysis of periodical and patent literature, reveals that oxides of iron, aluminum, vanadium, titanium, and other metals have been suggested for selective oxidation of H.sub.2 S to S. Such oxides display catalytic activity for H.sub.2 S oxidation, but they have not found wide application in selective oxidation processes because of their disadvantages. Iron oxide as a catalyst for H.sub.2 S oxidation was suggested by Claus about 100 years ago. However, the form of oxides proposed by Claus did not achieve high selectivity. In USSR Inventors Certificate 871873 iron oxide with small specific surface, reduced by calcination at a high temperature to turn Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4, is disclosed as being more selective than the iron oxide used by Claus. Use of a catalyst containing iron oxide is described in U.S. Pat. Nos. 4,576,925 and 4,519,992, as well as U.K. Patents Nos. 2,164,867A and 2,152,489A, all of which are incorporated herein by reference.
Aluminum oxide (Al.sub.2 O.sub.3) is also mentioned as a catalyst for H.sub.2 S oxidation, but has the disadvantage that it is catalytically active in the reverse Claus reaction (7). In addition, it is not stable and can lose its activity quickly because of surface sulfation.
Vanadium oxide, which is used in catalyst compositions for the Selectox process described in U.S. Pat. No. 4,311,683, has the disadvantage that it is very active for reactions (6) and (7), and therefore does not have a high selectivity for H.sub.2 S conversion to S.
Titanium oxide as a catalyst for H.sub.2 S oxidation to S has also been suggested. However, this oxide is catalytically active not only in reaction (4), but also for reaction (7). Thus, it can be used for selective oxidation of H.sub.2 S by oxygen only for low water content reaction mixtures.
Heterogeneous catalysts containing iron and chromium oxides for H.sub.2 S oxidation to S have been described, for example, in U.S. Pat. Nos. 4,818,740 and 4,576,925. More complex catalysts comprising three or more metal oxides have been described, for example, in UK Patent No. 2164867A. In addition to iron and chromium oxides, one of several oxides of the following metals were added in a quantity of 1.5-25% by weight: cobalt, nickel, manganese, copper, zinc and titanium. Although the addition of zinc and titanium oxide can improve the properties of an iron oxide based catalyst, nevertheless these catalysts display noticeable activity in the reverse Claus reaction and in the oxidation of sulfur to SO.sub.2.
Accordingly, there is a need for a highly efficient and highly selective catalyst that is effective in converting hydrogen sulfide to sulfur at temperatures above the sulfur dew point to about 300.degree. C.